This invention relates to hydrotreating a petroleum distillate. More particularly, this invention relates to a hydrotreating process comprising a plurality of hydrotreating zones operating at distinct operating conditions.
Refiners, to compete in the marketplace, have continuously sought to improve the quality of mid-distillate products while at the same time reducing processing costs. In the past few years, refiners have tended to shift away from the construction of new equipment for upgrading poorer stocks to the maximizing of existing equipment in order to achieve the desired upgrades. This has become a continual challenge to refiners as not only have stocks become heavier and generally poorer in quality, but also the threat of more stringent mandates on emissions and other burning qualities has further strained available upgrading capacity.
Regulations limiting the quantity of sulfur in distillate products are in effect in most parts of the world. The Environmental Protection Agency (EPA) on the federal level and various state agencies such as the California Air Resources Board (CARB) have envisioned more stringent standards for the future (0.05 wt % sulfur by 1994). The control of sulfur in refinery products is presently accomplished through hydrotreatment of full boiling range distillates to reach the sulfur specifications. The present invention describes a method to enhance the performance of hydrotreaters to remove more sulfur.
To better understand and appreciate the present invention, it is helpful to point out some of the limitations and drawbacks of current hydrotreating configurations. A hydrotreater typically consists of a single unit receiving feed from a single source, processing it to a single set of product quality specifications, and discharging it to usually a single end product. This simple type of system suffers from many limitations and inefficiencies. First of all, every molecule experiences the same nominal residence time in the reactor, without regard to its product quality deficiencies. Conceptually, this results in the over-treating of molecules already at or near product quality targets, while it results in underprocessing of difficult molecules to achieve a product which, on balance, is of acceptable quality. In the case of sulfur removal, much more desulfurization occurs at the inlet end of the reactor zone where easy mercaptans and thiophenes are reacting versus at the back of the bed where the harder benzothiophene species are reacting. Thus, the back of the bed is capable of removing more tons of sulfur per day if the concentration of easy sulfur species could be increased in the back of the reactor, for example, by increasing throughput. An analogous situation exists between olefin saturation and aromatics saturation for cetane improvement of the product. Olefin saturation occurs relatively easily near the front of the bed, while aromatics saturation is more difficult. In typical hydrotreating systems, lighter fractions with their olefins already saturated must ride through the back of the catalyst bed, thereby depriving the aromatics of important residence time needed to saturate rings and unnecessarily diluting the concentration of aromatics.
Secondly, with the uniform residence time for all molecules in a conventional hydrotreating design, less than optimal utilization of hydrogen occurs. After easy fractions have met product specifications in the front of the bed, they continue to absorb more hydrogen until they exit the catalyst bed.
Thirdly, because the product and its specifications are considered in total, it is difficult to take advantage of segregations in product qualities throughout the boiling range as they may exist. For example, the lighter cuts may be substantially free of heteroatoms, making them an acceptable higher valued product or feedstock for another catalyst which is sensitive to heteroatoms such as sulfur or nitrogen. Alternatively, cetane in the heavier cuts may be significantly higher than in the overall stream, reducing the need for premium streams as corrector stocks. The typical hydrotreating unit makes no attempt to sort hydrotreated molecules into different product classifications according to properties.
The combination of a hydrotreater and a fractionator or simple separation stage in an integrated process is generally known. For example, U.S. Pat. No. 3,806,444; U.S. Pat. No. 4,179,355; and U.S. Pat. No. 4,179,354 individually disclose separating the effluent from a hydrotreater as part of a process to increase the efficiency of sulfur removal. When treating heavy oils, a distillation tower may be used to dewax, as shown in U.S. Pat. No. 4,592,828. Both U.S Pat. No. 4,655,903 and 3,726,787 employ a distillation column as part of a process to remove polynuclear aromatics from the product when treating heavy oil feeds.
It is conventional to employ more than one reactor or reaction zones in a hydrotreating process. U.S. Pat. No. 3,728,249 to Antezano discloses two hydrotreaters in series, in which case at least a part of the feed passes through the second hydrotreating zone. The total composition of the feedstock to each of the zones necessarily differs. U.S. Pat. Nos. 3,607,723 Peck et al., 3,671,420 to Wilson et al., and 3,843,508 to Wilson et al. individually relate to split flow hydrodesulfurization in which there is an in-situ separation of the feedstock, and in which the two hydrotreating zones are intercommunicating sections of a column. A disadvantage of this approach is that essentially the same conditions, such as temperature and pressure in both hydrotreating zones are mandated by this arrangement. Also, the space velocities through each zone cannot be independently tailored.